Loki was born a frost giant runt, so he's actually around the same size as a normal human or a god. Because of this, his father basically treated him like crap. Odin led the Asgardians in a raid on his father's kingdom, killed Loki's father, and adopted Loki. Odin raised him alongside his son Thor, but Loki's natural tendency toward cunning and sorcery over more battle-oriented skills led the very warrior-centric Asgardians to look down on him. Additionally, Loki's own sense that he didn't belong and his inferiority complex compared to Thor grew over the years into full-blown jealousy of Thor and hatred of Odin for not treating him as well as Thor. Since around his adolescence, he's been doggedly attempting to both earn Odin's acceptance and take revenge on him, Thor, and the other Asgardians for perceived slights by taking over Asgard through various schemes and tricks.